Remote Sensing Concept
Raster Data
Image Data
An image is a digital picture or representation of an object. Image data consist only of numbers. Each number in an image file is a data file value. Data file values are sometimes referred to as pixels. The data file value is the measured brightness value of the pixel at a specific wavelength.
Raster image data are laid out in a grid similar to the squarges on a checkerboard. Each cell of the grid is represented by a pixel, also known as a grid cell.
In remotely sensed image data, each pixel represents an area of the Earth at a specific location. The data file value assigned to that pixel is the record of reflected radiation or emitted heat from the Earth’s surface at that location.
Bands
Each band is a set of data file values for a specific portion of the electromagnetic spectrum of reflected light or emitted heat (red, green, blue, near-infrared, infrared, thermal, and so forth) or some other user-defined information created by combining or enhancing the original bands, or creating new bands from other sources.
Numeral Types
The range and the type of numbers used in a raster layer determine how the layer is displayed and processed. For example, a layer of elevation data with values ranging from -51.257 to 553.401 would be treated differently from a layer using only two values to show land and water.
Coordinate Systems
The location of a pixel in a file or on a displayed or printed image is expressed using a coordinate system. There are two basic coordinate systems:
File Coordinates
File coordinates refer to the location of the pixels within the image (data) file. File coordinates for the pixel in the upper left corner of the image always begin at 0, 0.
Map Coordinates
Map coordinates may be expressed in one of a number of map coordinate or projection systems. The type of map coordinates used by a data file depends on the method used to create the file (remote sensing, scanning an existing map, and so forth).
